An MRI apparatus is an apparatus that measures NMR signals from nuclei, mainly protons, present in a biological object and images a nuclear density (proton density) or phase information of NMR signals in a biological tissue. In MRI apparatuses, various blood flow imaging methods using differences in behaviors between blood flow protons and stationary tissues have been developed, and technologies for drawing desired blood flows with higher luminance than other blood flows using differences in the directions of the blood flows have widely been used. For example, there are technologies for preliminarily exciting upstream or downstream blood flow protons and suppressing signals of blood flows entering imaged regions, and separating arteries from veins.
In such technologies, RF pulses saturating protons of unnecessary blood flows in advance and weakening signals (referred to as pre-saturation pulses) are used to suppress signals from the unnecessary blood flows. In the methods, arteries can be separated from veins, but any desired arteries or veins may not selectively be drawn among a plurality of arteries or veins. In contrast, a method (2D excitation method) of exciting only a desired part has been developed by combining RF pulses and region-selection gradient magnetic fields.
On the other hand, in current MRI apparatuses, RF pulses are radiated using multi-channel transmission coils in which a plurality of small coils are combined. In this case, for an excitation RF pulse, there is a request for desiring a magnetic field radiated to an excited region (radiated magnetic field) to be spatially uniform. This is because when a distribution is present in an excited magnetic field, irregularity occurs due to the distribution in an NMR signal from a tissue, and thus an accurate proton distribution or phase information may not be obtained. For this reason, in MRI apparatuses of the traditional way, a magnetic field distribution generated by an RF pulse radiated in advance from each channel is measured, a correction amount for uniformizing the magnetic field distribution is calculated, and this correction amount is superimposed on a driving voltage of an RF coil generating an RF pulse (see JP-A-2010-29640). This correction amount is referred to as an RF shimming parameter.